12. THE LEAF. 



29 



plete case, the phyllopodium is differentiated into three regions : a 

 basal portion, the leaf-base or hypopodium ; an apical portion, the 

 epipodium ; and an intermediate portion, the mesopodium, leaf- 

 stalk, or petiole ; but the last-named portion is frequently absent. 

 Most commonly the leaf assumes a flattened fcfrm in consequence 

 of the development of a relatively thin membranous icing along 

 one or other of these regions in the lateral plane : the epipodium 

 is typically winged, and then constitutes what is known as the 

 blade or lamina of the leaf ; the mesopodium is rarely winged, the 

 hypopodium more frequently so. 



The growth in length of the leaf is at first apical in all cases ; it 

 may continue to be apical (e.g. Ferns generally) ; or apical growth 

 may be early arrested, further elongation being effected by basal 

 growth (e.g. Iris, Onion, Myriophyllum, Poten- 

 tilla anserina) ; or, more rarely, basal and 

 apical (e.g. Achillea MillefoUum, and other 

 Composite) growth may occur simultaneously. 



A characteristic feature of leaves is that 

 their growth in length is limited ; but this is 

 not without exception ; in fact, there are all 

 intermediate forms between those which have 

 limited and those which have unlimited 

 growth. Thus, in most Phanerogams the 

 leaves have limited growth ; the cells of the 

 leaf are all actually formed at the time of its 

 unfolding, and all that takes place subsequently 

 is that the cells grow to their definitive size. 

 In a few of these plants, however, (e.g. Gruarea 

 and other Meliaceae) the pinnate leaves have 

 an apical growing-point by which new cells 

 are formed, and the growth in length of the leaf and the de- 

 velopment of lateral branches is carried on after the leaf has 

 unfolded. Long-continued apical growth appears to be the general 

 rule in Ferns : in Pteris aquilina and in Aspidium Filix-Mas 

 the leaf grows for three years ; and in Grleichenia, Lygodium, many 

 Hymenophyllacese, and Nephrolepis, the leaf grows for many years 

 after its appearance above the soil. The most striking example of 

 long-continued basal growth is that of the two leaves of Wel- 

 witschia which persist and grow basally as long as the plant lives, 

 and consequently attain a great length. 



The leaves are inserted upon the nodes (p. 11) of the stem, the 



FIG. 18.-Leafof Ban- 

 unculus Ficaria: v leaf- 

 base (hypopodium) ; p 

 petiole (mesapodium) ; 

 t lamina (epipodium). 



